The present invention relates to an optical head apparatus for writing data in and reading date from an optical disk, a form of an optical information recording medium, and particularly to an improvement for minimizing the astigmatism of a converging optical system in an optical head having a converging lens formed by molding.
The apparatus of this type are shown for example in the following publications:
(a) Japanese Patent Application Laying-open No. 102342/1983 PA1 (b) Modern Optical Engineering, McGraw-Hill, N.Y. 1966 PA1 (c) Technique for Designing a Lens, pp. 35-36, Kogaku Kogyo Gijutsu Kyokai, Japan PA1 (d) Optics, p. 156, Wiley, N.Y. PA1 (e) The Present Status of Plastic Lens, Harada, Journal of the Institute of Television Engineers of Japan, Vol. 38, No. 9 (1984), pp. 810-814. PA1 (f) Japanese Patent Application Laying-open No. 50335/1982
In addition, a device shown in FIG. 3a and FIG. 3b was known, in which there is provided a light source such as a semiconductor laser 40 which emits a linearly polarized light pencil 2. A diffraction grating 3 splits the incident light pencil (alternatively referred to as a light beam) 2 into three pencils of light. A half prism 4 separates the illuminating light pencil 5 and the reflected light pencil 6 from each other. A collimator lens 7 converts the illuminating light 5 into parallel light pencil 8. A reflection prism 9 reflects the parallel light pencil 8 to a direction substantially at right angles. A converging lens 10 converges the parallel light pencil 11 toward an information track 13 of a disk-shaped recording medium or carrier 12 to form a light spots 14. For this to be achieved, the recording medium 12 is positioned at or close to the focal point of the converging lens 10. To be more exact, the light spots 14 consists of three spots 14a through 14c as shown in FIG. 3b. The information track 13, on the other hand, consists of pits 15 and lands 16. The recording medium 12 is rotated by a drive motor not shown. The light pencil reflected at the recording medium 12 passes through the converging lens 10, and the collimator lens 7 and is reflected at the half prism 4 substantially at right angles. The reflected light pencil 6 passes a concave lens 17 for reducing the angle of convergence of the reflected light pencil. A cylindrical concave lens 18 causes an astigmatism in the light pencil having passed the concave lens 17. A light detector 19 comprises detecting elements 19a through 19c.
The light pencil 2 emitted from the semiconductor laser 40 is linearly polarized in a direction parallel to its PN junction. The light pencil 2 passes through the diffraction grating 3, a half prism 4, and a collimator lens 7, and is deflected by the reflection prism 9, and is converged, forming three beams 14a through 14c, on the recording medium 12. The light pencil as reflected on the recording medium passes through the converging lens 10 and is again reflected at the reflection prism 9, and is then reflected at the half prism 4. The light pencil then passes the concave lens 17 and the cylindrical lens 18 and enters the three detecting elements 19a through 19c of the light detector 19. The central detector 19a receives the reflected light from the light spot 14a, and converts the intensity of the received light into an electrical signal. The intensity of the reflected light differs depending on whether the light spot 11 is reflected in the pit or on the land 16. The electrical signal is used for an audio signal, a video signal, digital data, or the like.
As the recording medium 12 rotates, the surface of the recording medium 12 may vary in position in the direction of optical axis of the object lens 10 because of undulation, vibration and the like. The deviation in the direction of the optical axis from the focal point is detected in a known manner (literature (a)) in accordance with the variation of the shape of the light pencil on the central photodetector 19a, and is corrected by a servo mechanism, not shown, so that the surface of the disc is kept at the foal point.
The rotation of the recording medium also causes relative deviation in the lateral direction between the central beam 14a and the track 13, due to meandering of the track 13 and vibration. To correct the error in the lateral direction, the difference between the outputs of the photodetectors 19b and 19c is detected and used as a representation of the deviation between the track and the spot 14a (which is the relative deviation of the track), and correction is made in accordance with the detected deviation (see the literature (a) mentioned above).
To maximize the density of information stored on the recording medium in such an optical head apparatus, the pit length and the track pitch are made to have the smallest size which allows reading when the converging system from the semiconductor laser to the converging lens is in the ideal state of the diffraction limit. Typically, when the laser wavelength .lambda.=780 nm, and the numerical aperture of the converging lens at the side of the disc NA=0.5, then the spot diameter obtained by converging at the diffraction limit is about .lambda./NA=1.6 microns. The track pitch is therefore 1.6 microns, while the minimum pit length is 0.8 microns, half the minimum spot diameter.
In order for the converging system to have the characteristic of the diffraction limit, it is required that
(1) the light emitted from the semiconductor laser 40 should be conducted at or close to the state of stigmatism throughout the entire light path passing the converging lens up to the spot 14, and
(2) the semiconductor laser itself does not have any aberration.
In recent years, molded plastic lenses are used as the converging lens of the optical head apparatus. This is along the line of the advancement in the plastic precision molding technique. The plastic lenses can be formed to have aspheric surfaces, if the die is machined by precision NC (numerical controlled) machines. This is in contrast to glass lenses which have to be polished individually and which therefore were generally restricted to spherical surfaces. It is therefore possible to replace conventional combination of three to five spherical glass lenses by a single aspherical plastic lens.
Because the plastic lens can be fabricated by plastic molding which is suitable for mass production, and because the number of lenses can be reduced, the use of the plastic lens contributes to the costdown of the optical head and reduction in the number of assembly steps of the optical head, so that the plastic lenses will be increasingly used in the future design of optical heads.
A disadvantage associated with the plastic lens is the lack of uniformity in the aberration characteristic along the direction of rotation about the optical axis of the lens. More particularly, it has been found that the astigmatism of the molded plastic lens against linearly polarized light varies when the lens is rotated about the optical axis.